Orbital sanders, such as random orbital sanders, are used in a variety of applications where it is desirable to obtain an extremely smooth surface free of scratches and swirl marks. Such applications typically involve wood working applications such as furniture construction or vehicle body repair applications, just to name a few.
Random orbital sanders typically include a platen that is driven rotationally by a motor-driven spindle. The platen is driven via a freely rotatable bearing that is eccentrically mounted on the end of the drive spindle. Rotation of the drive spindle causes the platen to orbit about the drive spindle while frictional forces within the bearing, as well as varying frictional loads on the sanding disc attached to the platen, cause the platen to also rotate about the eccentric bearing, thereby imparting the “random” orbital movement to the platen. Typically such random orbit sanders also include a fan member which is driven by the output shaft of the motor. The fan member is adapted to draw dust and debris generated by the sanding action up through openings formed in the platen and into a filter or other like dust collecting receptacle.
One such prior art random orbital sander is disclosed in U.S. Pat. No. 5,392,568 for Random Orbit Sander Having Braking Member (the entire disclosure of which is incorporated herein by reference). For context, a short section of the '568 patent describing a random orbital sander is repeated here. With reference to FIG. 7, a random orbital sander 10 generally includes a housing 12 which includes a two-piece upper housing section 13 and a two-piece shroud 14 at a lower end thereof. Removably secured to the shroud 14 is a dust canister 16 for collecting dust and other particulate matter generated by the sander during use. A platen 18 having a piece of sandpaper 19 (FIG. 8) releasably adhered thereto is disposed beneath the shroud 14. The platen 18 is adapted to be driven rotationally and in a random orbital pattern by a motor disposed within the upper housing 13. The motor (shown in FIG. 8) is turned on and off by a suitable on/off switch 20 which can be controlled easily with a finger of one hand while grasping the upper end portion 22 of the sander. The upper end portion 22 further includes an opening 24 formed circumferentially opposite that of the switch 20 through which a power cord 26 extends.
The shroud 14 is preferably rotatably coupled to the upper housing section 13 so that the shroud 14, and hence the position of the dust canister 16, can be adjusted for the convenience of the operator. The shroud section 14 further includes a plurality of openings 28 (only one of which is visible in FIG. 7) for allowing a cooling fan driven by the motor within the sander to expel air drawn into and along the interior area of the housing 12 to help cool the motor.
With reference now to FIG. 8, the motor can be seen and is designated generally by reference numeral 30. The motor 30 includes an armature 32 having an output shaft 34 associated therewith. The output shaft or drive spindle 34 is coupled to a combined motor cooling and dust collection fan 36. In particular, fan 36 comprises a disc-shaped member having impeller blades formed on both its top and bottom surfaces. The impeller blades 36a formed on the top surface serve as the cooling fan for the motor, and the impeller blades 36b formed on the bottom surface serve as the dust collection fan for the dust collection system. Openings 18a formed in the platen 18 allow the fan 36b to draw sanding dust up through aligned openings 19a in the sandpaper 19 into the dust canister 16 to thus help keep the work surface clear of sanding dust. The platen 18 is secured to a bearing retainer 40 via a plurality of threaded screws 38 (only one of which is visible in FIG. 8) which extend through openings 18b in the platen 18. The bearing retainer 40 carries a bearing 42 that is journalled to an eccentric arbor 36c formed on the bottom of the fan member 36. The bearing assembly is secured to the arbor 36c via a threaded screw 44 and a washer 46. It will be noted that the bearing 42 is disposed eccentrically to the output shaft 34 of the motor, which thus imparts an orbital motion to the platen 18 as the platen 18 is driven rotationally by the motor 30.
With further reference to FIG. 8, a braking member 48 is disposed between a lower surface 50 of the shroud 14 and an upper surface 52 of the platen 18. The braking member 48 comprises an annular ring-like sealing member which effectively seals the small axial distance between the lower surface 50 of the shroud 14 and the upper surface 52 of the platen 18, which typically is on the order of 3 mm .+−.0.7 mm.
With reference to FIG. 9, the braking member 48 includes a base portion 54 having a generally planar upper surface 56, a groove 58 formed about the outer circumference of the base portion 54, a flexible, outwardly flaring wall portion 60 having a cross sectional thickness of preferably about 0.15 mm, and an enlarged outermost edge portion 62. The groove 58 engages an edge portion 64 of an inwardly extending lip portion 66 of the shroud 14 which secures the braking member 48 to the lip portion 66. In FIGS. 8 and 9, the outermost edge portion 62 is illustrated as riding on an optional metallic, and preferably stainless steel, annular ring 61 which is secured to the backside 52 of the platen 18. Alternatively, the entire backside of the platen 18 may be covered with a metallic or stainless steel sheet. While optional, the stainless steel annular ring or sheet 61 serves to substantially eliminate the wear that might be experienced on the upper surface 52 of the platen 18 if the outermost edge portion 62 were to ride directly thereon.
With brief reference to FIG. 10, the braking member 48 further includes a pair of radially opposed tabs 68 which engage notched recesses 70 in the inwardly extending lip portion 66 of the shroud 14. This prevents the braking member 48 from rotating with the platen 18 relative to the shroud 14 during operation of the sander 10. The braking member 48 is formed by injection molding as a single component from a material which allows a degree of flexure of the wall portion 60, and preferably from polyester butylene terephthalate (hereinafter “PBT”).
The operation of the braking member 48 during use of the sander 10 will now be described. As the platen 18 is driven rotationally by the output shaft 34 of the motor 30, the outermost edge portion 62 of the braking member 48 rides frictionally over the upper surface 52 of the platen 18. The outermost edge portion 62 of the braking member 48 exerts a relatively constant, small downward spring force onto the stainless steel ring 61. The spring force is such that the random orbital action of the platen 18 is substantially unaffected under normal loading conditions, but the rotational speed of the platen 18 is limited when the platen 18 is lifted off of the work surface to about 1200 rpm. It has been determined that an operating speed of at least about 800 rpm is desirable to prevent the formation of swirl marks on the surface of the workpiece when the platen is loaded. Thus, 800 rpm represents a preferred lower speed limit which the braking member 48 must allow the platen 18 to attain when engaged with a work surface during normal operation to achieve satisfactory sanding performance. It has further been determined that if the platen is permitted when unloaded to attain rotational speeds substantially above normal operating speeds-e.g., above approximately 1200 rpm—the rapid deceleration that results when the platen is reapplied to the workpiece causes the sander 10 to jump which can produce undesirable gouges or scratches in a work surface. Thus, it is desirable for the braking member 48 to prevent the rotational speed of the platen 18 about bearing 42 to exceed approximately 1200 rpm when the platen 18 is unloaded, and permit the platen 18 to rotate above approximately 800 rpm when loaded.
To achieve the desired braking action the braking member 48 exerts a relatively constant preferred braking force of about 3.5 lbs. onto the stainless steel ring 61 at all times during operation of the sander 10. This degree of braking force is significantly less than the frictional torque imposed by the interface of the sandpaper 19 secured to the platen 18 and the workpiece, but of the same order of magnitude as the torque applied by the bearing 42. Consequently, the brake member 48 has an insignificant effect on the normal operation of the platen when under load, and a speed limiting effect on the platen when unloaded.
The desired braking force of about 3.5 lbs. is achieved by the combination of the geometry of the braking member 48 as well as the material used in its formation. It has been found that the use of PBT doped with about 2% silicon and about 15% Teflon provides a preferred flex modulus of about 46.5 kpsi. However, a material which provides a flex modulus anywhere within about 35 kpsi to 75 kpsi should be suitable to provide the desired degree of flexure to the brake member 48. The amount of braking force generated by the braking member 48 is important because a constant braking force in excess of about 4 lbs. causes excessive wear at the outermost edge portion 62, while a braking force of less than about 3 lbs. is too small to appropriately limit the increase in rotational speed of the platen 18 when the platen 18 is lifted off of a work surface.
One disadvantage the electrically powered random orbital sanders have compared to pneumatic sanders is due to the height of the sander. Heretofore, electrically powered random orbital sanders and orbital sanders have used mechanically commutated motors, such as universal series motors in the case of corded sanders, which dictates that the overall height of the electrically powered sander is greater than a comparable pneumatic sander. In electrically powered random orbital sanders, if the user grasps the sander by placing the palm of the user's hand over the top of the sander, the user's hand is sufficiently far from the work that the user is sanding to cause more fatigue than is the case with pneumatic sanders where the user can grasp the sander close to the work piece. This often leads to user's grasping electrically powered random orbital sanders on the side of the sander. This tends to be awkward compared to grasping the top of the housing. Also, the greater height of the electrically powered random orbital sander causes more wobble compared to the lower height pneumatic random orbital sander. The electrically powered sander is heavier than a comparable pneumatic sander due to the weight of the motor, further contributing to the wobble problem. The user of the electrically powered random orbital sander thus must grasp it more tightly than the lower height and weight pneumatic random orbital sander, causing additional fatigue in the user's hand.